The invention relates to lasers and is particularly directed to a feedback control system for, firstly, general offset frequency locking of a slave laser to a reference laser by phase locking the beat frequency between the lasers to an offset reference frequency and, secondly, stabilizing the light frequency of a laser subject to retro-reflection through the use of a frequency offset locking technique.
The parent application showed apparatus and a method for establishing a highly stable single-frequency laser output by phase locking the beat frequency signal in a transverse Zeeman laser to the output of a crystal-controlled frequency synthesizer. It also described art relevant to that apparatus and method. The problem of retro-reflection was pointed out and an illustration given of a method for identifying sources of such retro-reflection so that appropriate precautions could be taken for its reduction. The effects of retro-reflection on frequency stability are quite significant having been observed by many investigators [e.g. W.R.C. Rowley, I.E.E.E. Tras. Instrum. and Meas. I.M. 15, 146 (1966)]. It shows up in Lamb-dip stabilized lasers [A. L. Bloom and D. L. Wright, Appl. Opt. 5, 1528 (1966)]and even in iodine stabilized lasers [K. Tanaka and T. Kurosawa, Japanese J. Appl. Phys. 15, 2271 (1976)].
A well-known technique for reducing retro-reflection from a specular source is to use a polarizer and a quarter waveplate with its axis set at 45.degree. so that the retro-reflected light which traverses the waveplate twice undergoes a half wave retardation at 45.degree. and is therefore in effect rotated by 90.degree. and subsequently blocked by the polarizer. Reductions of retro-reflection of the order of 10.sup.-5 to 10.sup.-6 are obtainable this way with a V-coated waveplate. The residual effect stems largely from a zig-zag double internal reflection in the quarter waveplate which contributes a component with additional half wave retardation such that this component is not rejected by the polarizer, no matter what its extinction ratio. Faraday rotators can be used, but they are expensive and provide much less attenuation. They are, however, effective in suppression of back scattered light, unlike the polarizer and quarter waveplate combination. Yet, in some situations--for example, retro-reflection occurring in fibre optic applications--retro-reflection is difficult or almost impossible to handle adequately because phase fluctuations cannot effectively be canceled by the aforementioned techniques. There is, nonetheless, one technique that can be used to provide an extraordinary degree of frequency stability in the face of substantial retro-reflection. From a survey of the literature it seems not to have been applied previously, perhaps because of the apparently insurmountable complexity and high cost involved The technique is based on the slaving, to a reference by the method of offset frequency locking as herein described, the laser subjected to retro-reflection. The requirements of this technique are made particularly simple and cost effective by the disclosures of the parent invention.
L. H. Enloe et al [U.S. Pat. No. 3,437,955 (1969)]purport to disclose a technique for locking the phase of one laser with another (i.e. a zero frequency offset homodyne system), but the practical difficulties in operating such a system are formidable: there is first the problem of setting up the right conditions for lock acquisition, in view of the fact that the loop tracking range the Enloe et al. system is only 1 part in 5000 of the normal laser frequency excursions, and unless the beat frequency is observed with an oscilloscope or frequency counter, the correct condition is not found easily by manual control. Secondly, there is the problem of the intrinsic phase jitter of the 6328 neon line in particular, even under the most favorable conditions [investigated by A. E. Siegman et al, I.E.E.E. J. Quantum Electron. QE-3, 180 (1967)]. This second problem, combined with that of establishing a resonant frequency in the piezo transducer [see P. W. Smith, I.E.E.E. J. Quantum Electron. QE-1, 343 (1965)]sufficiently high to keep the loop phase shift below the critical 90.degree. limit over the jitter frequency range, makes true phase locking particularly difficult. Enloe et al suggest in a concluding paragraph that their techniques can further be used to achieve the offset frequency locking of two lasers by disposing a second phase detector between the photodetector and transducer so that the beat frequency generated in the former can be compared with a reference difference (off-set) frequency. In that case, however, the foregoing problems are compounded by two additional ones stemming from the then-known types of phase detectors: mirror image locking, and harmonic locking. These problems make possible a multiplicity of lock frequencies above and below the reference laser frequency. In Enloe et al, neither the recognition of these problems nor their solution is offered.
A frequency offset locking technique for two lasers was first practiced by J. L. Hall [I.E.E.E. J. Quantum Electron. QE-4, 638 (1968)], who used a frequency-to-voltage converter and analog servo loop to establish the frequency offset. In such a system there is no problem of locking on harmonics of the reference offset frequency and locking at an image frequency of the desired offset frequency is not possible, for if the active or "slave" laser is set to operate above the reference by the offset amount, it cannot lock with this offset below the reference laser because that condition does not lead to a stable servo loop. In Hall's system, long term frequency stability of the offset laser depends on the accuracy of the frequency-to-voltage conversion. While this can be made very high through the use of a frequency counter and digital-to-analogue converter [see H. Gerhardt and A. Timmerman, Opt. Comm. 21, 343 (1977)], improved performance still could be expected from phase locking the beat frequency between the two lasers to the desired offset frequency. This technique has been used by Hall [ see J. L. Hall in Fundamental and Applied Laser Physics, Proceedings, 1971 Esfahan Symposium, M.S. Feld, A. Javan, and N. A. Kurnit, Eds. (Wiley, New York, 1973), p. 463]and others [J. Helmcke, S. A. Lee, and J. L. Hall, Appl. Opt. 21, 1686 (1982)], but the precise details of their phase locking circuitry were not disclosed there and have evidently not been published.
Currently, D. A. Lewis and R. M. Evans (Iowa State University at Ames, Iowa) have published in Jan. 1985, in the `Review of Scientific Instruments` an article entitled `Laser Frequency Offset Synthesizer` dealing with their phase locking method for generating a laser frequency offset by a given amount from a reference laser frequency. The method of Lewis and Evans is clearly and substantially different from the compact method outlined in the final paragraph of my parent application. Likewise, there is inferential evidence that the invention of the parent application has not been employed in the work of Hall and others.